Coating of aluminium and/or aluminium alloys on steel surfaces



United States Patent 3,468,770 COATING 0F ALUMINIUM AND/0R ALUMINIUM ALLOYS 0N STEEL SURFACES Tarun Kumar Ghose, Iadavpur University, Calcutta 32,

India, and Ranendra Dutta, 28 Townshend Road, Calcutta 25, India N0 Drawing. Filed Dec. 22, 1965, Ser. No. 515,716 Int. Cl. C23c 3/00; C23b 7/00; C23f 17/00 US. Cl. 204-38 14 Claims ABSTRACT OF THE DISCLOSURE A flux composition for applying aluminium or aluminium alloy coatings to steel surfaces and a process for coating steel surfaces with aluminium or aluminium alloys and steel surfaces coated thereby.

STATE OF THE ART This invention relates to a composition used in a process for the coating of aluminium or aluminium alloy on steel surfaces.

Various methods have been suggested for the coating of aluminium on a steel surface. Such methods include the steps of passing the cleaned steel surface through a molten fluxing agent at a high temperature and thereafter in a bath of molten aluminium. Another method comprises the step of chemical passivation of the steel surface prior to the coating and which thus avoids the step of fluxing the cleaned steel surface. It has been found that such processes have not proved to be satisfactory and particularly with regard to a coated surface when used as a food container.

OBJECTS OF THE INVENTION It is an object of this invention to provide an improved composition used in the process of coating aluminium or aluminium alloy on steel surfaces.

It is a further object of this invention to provide a process for the preparation of a composition used in the process for the coating of aluminium or aluminium alloy on steel surfaces.

Still another object of this invention is to provide a process for the coating of steel surfaces with aluminium or aluminium alloy.

THE INVENTION According to this invention there is provided a composition used in a process for the coating of aluminium or aluminium alloy on steel surfaces, said coated surfaces being used for example as containers for food including processed food characterized in that the composition consists of a plurality of metal compounds, said composition being in a suspended state either in an aqueous system having therein 0 to 3% of a suitable emulsifying agent or in a pulverized state when in conjunction with a wetting agent being an inert medium, said metal compounds being selected from the groups such that the said composition provides a metallurgical bond between aluminium or aluminium alloy and a cleaned steel surface.

In accordance with one embodiment of the invention, the composition comprises chlorides of sodium, lithium, and potassium, aluminium fluoride, and zinc oxide.

Further in accordance with this invention the composition comprises chlorides of sodium, lithium, and potassium, aluminium fluoride, zinc oxide and a suitable modifier or modifiers and O to of zinc chloride.

The modifiers that may be employed in the composition are potassium bifluoride, and potassium fluotitanate.

Further, in accordance with this invention there is pro- 3,468,770 Patented Sept. 23, 1969 vided a composition comprising 1030% of sodium chloride, 5-25% of lithium chloride, 1070% of sodium fluoride, 5-40% of potassium chloride, .05-5% of zinc oxide, 540% of aluminium fluoride, and a modifier or modifiers.

Though trace amounts of the modifiers may be used in this invention, it is also within the scope of the invention to employ in the composition 05%, of potassium fluotitanate, and/0r 065% of potassium bifiuoride.

Further, according to this invention, there is provided a process for the coating of steel surfaces with aluminium or aluminium alloy which comprises the steps of applying the said composition in a suspended state on the said cleaned steel surface, drying said surface and thereafter applying the molten aluminium or aluminium alloy on said dried surface.

It would be appreciated that the application of the fiuxing agent should be applied on the cleaned steel surface at such a temperature which would not thereby result in an effective loss of certain properties of the flux. Preferably, the flux should be applied on the cleaned steel surface at room temperature.

The steel surface may be cleaned in any conventional manner and does not form a novel aspect of this invention. However, it is important that the steel surface must be in a cleaned state prior to the application of the fluxing agent as any foreign particles adhering to the surface would have a detrimental effect on the coating.

Subsequent to the application of the fluxing agent, the steel surface is dried for removal of water from the surface thereof and thereafter passed through a bath containing molten aluminium or aluminium alloy.

The coated steel surface may thereafter be anodized. The step of anodizing does not form an essential part of the process to provide the coated surface and may be carried out only in certain instances where certain particular properties of the coated surface are required. The electrolyte bath can consist of for example sulphuric acid and oxalic acid, the coated surface forming the anode and the cathode being usually formed of lead or aluminium electrode. The time required for the electrolytic deposition should preferably not exceed 15 minutes.

Reference is now made to the coating consisting of aluminium or aluminium alloy. Though aluminium does form a suitable coating on the steel surface, it is preferable to employ an aluminium alloy in order to minimize the costs and obtain improved properties of the coated steel surface. Types of alloys that may be used within the scope of this invention are for example:

aluminium titanium aluminium manganese aluminium silicon aluminium silicon titanium aluminium magnesium As is obvious to anyone skilled in the art, the alloys which should not be used for the purpose of a coating are those alloys which do not provide a good metallurgical bond and which are likely to be toxic for various usage of the product.

EXAMPLE 1 Mild steel base plate as supplied by Tin Plate Company of India Limited was obtained. The analysis of the base plate is given below. This type of base plates used in the hot dip tinning for the manufacture of tin plates suitable for the use in the food cans.

Analysis: C, 0.00060.1%; Mn, GAO-0.60%; P, 0.03- 0.05%; S, 0.03O.7%; Si, trace.

Thickness of the plate: 0.25-30 mm.; cold rolled and annealed.

The surface of the black plate was cleaned and superficial rust and corrosion was removed. Small pieces were sheared from the large plate, and after mechanical cleaning it was dipped in 10% hydrochloric acid solution after degreasing the plates in mild alkali solution containing Teepol surface active agent. The experimental pieces were kept immersed fully inside the acidulated water in the beaker till the time of actual fluxing and dipping in molten aluminium bath. It was observed that if the surface of the plate was not properly cleaned in alkali and acid solution in the hot dip process, it used to interfere and invariably a patchy alloying took place.

Dipping in the aluminium and molten aluminium alloys: Prior to actual hot dipping operation pellet tests were carried out with the composition or the flux as described above and this was used to give an indication whether the surface of the plate was clean enough and about the flow of the alloy materials.

In the first series of experiments two small crucibles made of refractory material were used, one for the melting of the flux and the other one for the melting of aluminium. They were put in a cold fire furnace fitted with blowers. Within half an hours time the metal melted and the flux similarly melted also in about 10 minutes time. The flux reacted vigorously with the refractory material at the molten stage leading to perforation of the crucible. This was expected since the fluoride content of the flux was in the molten stage. Small pieces of base plate were dipped first and then immediately dipped in the molten aluminium for nearly 20 seconds. The temperature of the commercially pure aluminium was kept at approximately 700 C. The plates could be thus aluminized but for the bending test it was found that the layer peeled off partially and it was observed that while the diffusion took place when inside the fluid it made the plate brittle.

At this stage the temperature of the aluminised plate was reduced after aluminising step, the plates were dipped in water to cool it down to room temperature quickly and thus the speed of diffusion was checked. A few strips were aluminised with commercially pure aluminium.

EXAMPLE II The same operation was carried out in this hot dip process except that the aluminium alloy was changed with the idea of reducing the effective dipping tempera ture and to have the advantage of plastic range of the alloy. An alloy having a comparatively low viscosity was used in this test and which thereby assisted in draining off the liquid metal from the plate. The fluxing method was also changed and only after slight warming of the plates, powder flux was sprinkled on the plate uniformly and then heated to bring it to the molten stage and then immediately dipped into the alloy at a temperature of about 670 C. Satisfactory aluminised surface was obtained.

EXAMPLE III Performance of aluminised steel panels in contact with different foods processed in fully lacquered cans In different series of experiments pack trials were undertaken with the following types of coated plates.

(1) Aluminised (2) Aluminised anodised (3) Aluminised lacquered (4) Aluminised anodised and lacquered (5) Plain tin container as comparison In total nearly 500 containers.

Usual methods of processing and canning of food were followed. The products packed were as follows:

Spinach Rice with pulses (spiced) Tomato juice Meat (mutton corn) Beet in brine Potato in brine Curried vegetables (spiced) Butter with salt Vinegar Tap water Mangoe in syrup In the first series of experiments aluminised steel panel of 5 cm. x 5 cm. size were cleaned and the cut edges were covered with a vinyl lacquer and stoved to remove the solvent, and for curing. Some of the panels were anodised in 15% sulphuric acid solution for about 10 to 15 minutes. These were also included as stated above in the pack trial. Aluminised panels were also coated with two coatings of universal food lacquer by dipping and draining method and stoved at 200 C.

Food lacquer Weiderhold WW7 was manufactured by M/s. Hermann Weiderhold, Germany. Some of the aluminised anodised panels were coated with universal lacquer. Adhesion to aluminised surfaces and aluminised anodised surface were satisfactory and there was no sign of peeling off and the coating thickness was not much in excess.

In the first series rice, peas, grapefruit, tomato and minced meat were processedand packed in fully lacquered jam cans. The following observations were made, after opening from a period of one week to 26 weeks of storage at room temperature. The storage period covered the summer months of the year. In the case of rice, meat, tomato, peas normally gives rise to excessive sulphur staining associated with detinning in a plain tin can (Without lacquer). Aluminised surface has been found in our experiments to have no surface corrosion staining or other type of chemical conversion of surface layer on aluminium. This was due to the surface oxidation of aluminium which had attained While it was aluminised. In the case of acid fruit packs like tomato and grapefruit juices aluminised anodised surface had shown better resistance to corrosion than aluminised alone and lacquered panels had shown marked improvement as well as corrosion resistance is concerned. In the case of highly acid packs at 2.5 to 3 pH there was a tendency of hydrogen swell in the cans with plain aluminised panels and aluminised anodised panels.

In the second series of experiments wide varieties of food products, such as meat, fish, vegetable and fruits were packed as described above, in panels of 10 cm. x 7.5 cm. sizes of different descriptions and as described above were inserted inside the cans with products and the results are summarise below:

Rice, meat, fish, spinach, tap water, beet, potato, peas, milk, dalda, butter and peas had shown no sign of staining of surface on aluminised anodised, aluminised lacquered and aluminised anodised lacquered plates. The products, after opening the cans at intervals between one week to 26 weeks have been tasted by different individuals and in no occasion had been found to have lost the wholesomeness. Acid fruit packs such as grapefruit, tomato juice, mangoe in brine, mangoe in syrup, etc., have been found to have various types of results. In the case of grapefruit juices it was found to have the minimum corrosion on aluminised and aluminised anodised panel and in the other highly acid packs hydrogen swells have been observed, but with the lacquered panels the acid packs had relatively better corrosion resistance. The control packs i.e. the plain tin cans have also been examined in the products such as fish, meat, rice, potato, etc., leave sulphur staining and de-tinning had occurred and the products were also partly coloured at the contact surface.

A few remarkable effects were noticed.

In the case of beet in brine, the product has been found to have no change in colour in the packs containing aluminised panels whereas in the tin plate cans the product was fully bleached to discolouration.

The aluminised steel surface achieved in the process obtained above has been found to have the property of corrision resistance in humid atmospheric conditions or exposed to water better than that of galvanised steel surfaces in the fields of applications such as structural sheets, general containers and pipes, pipe fittings and joints.

We claim:

1. A flux composition for use in a process for the coating of cleaned steel surfaces with a metal selected from the group consisting of aluminium and aluminium alloy consisting essentially of 1030% of sodium chloride, 525% of lithium chloride, 10-70% of sodium fluoride, 5-40% of potassium chloride, .055% of zinc oxide and 530% of aluminium fluoride.

2. A composition of claim 1 which includes at least one modifier selected from the group consisting of potassium bifiuoride and potassium fiuotitanate.

3. A composition of claim 2 wherein the amount of potassium bifluoride is to 65% and the amount of potassium fiuotitanate is 0 to 5%.

4. A composition of claim 2 containing 0 to 3% of an emulsifying agent.

5. A composition of claim 1 containing 0 to of zinc chloride.

6. A composition of claim 5 wherein the elements of the said composition are in an aqueous suspended state.

7. A composition of claim 5 wherein the composition contains an inert medium.

8. A process for the coating of steel surfaces with aluminium or aluminium alloy which comprises the steps of applying a layer of a composition of claim 1 onto a cleaned steel surface, drying said surface and then applying molten aluminium or aluminium alloy onto said dried surface.

9. A process of claim 8, wherein said layer is a film of the said composition.

10. A process of claim 8 wherein the said composition applied to the cleaned steel surface is a composition in an aqueous suspension.

11. A process of claim 8 wherein the said composition is in powder form and is sprayed on the cleaned steel surface in conjunction with an inert medium.

12. A process of claim 8 wherein said cleaned steel surface is coated with said composition at room temperature.

13. A process of claim 8 wherein said coated surface is cooled after applying the aluminium or aluminium alloy coating.

14. A process of claim 13 wherein said coated surface is anodised in an electrolytic bath.

References Cited UNITED STATES PATENTS 2,101,553 12/1937 Mattsson. 2,671,737 3/ 1954 Jominy et al 117-51 2,785,084 3/ 1957 Lundin 117-51 3,027,269 3/ 1962 Teshima et al l1751 FOREIGN PATENTS 359,495 10/ 1931 Great Britain.

OTHER REFERENCES Horii: Chemical Abstracts, vol. 55, 1961, p. 1398.

Einerl et al.: Chemical Age, vol. 46, 1942, pp. 181-183, 235-236.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

